This invention relates to a fuel vapor handling system. More particularly, the invention is directed to an apparatus and method for use in diagnosing the system. The subject of this application is related to the following copending patent application: Ser. No. 08/236,071, filed May 2, 1994, entitled Conductivity Sensor Diagnostic for Fuel Vapor Handling System, filed concurrently with this specification and assigned to the assignee of this invention.
Automobiles conventionally include a system directed to controlling the emission of fuel vapors generated by fuel carried in the vehicle's fuel system. These evaporative emission control systems, known as "EVAP" systems, are implemented as a collateral system to the fuel system. The diurnal and running loss vapors the EVAP system collects result primarily from ambient temperature excursions and from the cyclic operation and parking of a vehicle that results from the operator's use of the vehicle as transportation.
An EVAP system typically includes a vapor collection system with an adsorption mechanism to capture and store vapors generated by the fuel system. The EVAP system also includes a purge system to transfer the stored vapors from the adsorbent to the vehicle's engine for consumption in the normal combustion process. The purge system generally includes a purge valve that selectively opens a passage between the EVAP system and the vehicle's engine to effect a controllable rate of purge.
Conventionally, effective diagnosis of an EVAP system is generally provided through manual inspection of the system in response to noticeable engine performance degradation or noticeable fuel or vapor leakage. Periodic manual vacuum testing for leaks and purge valve functional checking provides additional effectiveness in diagnosing system operation.
Research has been conducted into developing on-board means for automatically diagnosing EVAP systems, capable of automatically detecting leaks in the system and determining whether the purge system is operating properly. Development of on-board automatic diagnostic systems has generally resulted in proposed systems related to mechanisms that close the EVAP system off from the atmosphere and then generate a positive or negative internal system pressure. By then measuring changes in the system pressure, the diagnostic mechanisms attempt to discern whether the evaporative control system is functioning properly.
Generally, sensitive diagnostic systems are proposed with precision pressure detection devices to work on small pressure differentials. To avoid unacceptable erroneous fault reporting, a pressure based diagnostic system must be able to discern that unexpected pressure gradients are a result of system malfunctions and not changing ambient conditions or other normal collateral effects. This tends to complicate and drive up the cost of a diagnostic mechanism. Accordingly, automatic EVAP diagnostic systems have proven difficult to implement.
Adsorption canister collection and storage system use in on-board refueling vapor recovery (ORVR) systems is known. ORVR systems are vehicle based systems directed to capturing fuel vapors generated by the transfer of fuel from a pump to a vehicle. ORVR systems have been proposed that are configured in a manner similar to an EVAP system including a storage canister and a purge system.